The production of a metal capacitor, wherein the metal is capable of forming a stable oxide layer having a dielectric constant greater than 25 (k≧25), preferably selected from the group consisting of tantalum and niobium, comprises some common general steps. Just for the sake of simplicity, the description hereunder relates to tantalum: (a) mechanical pressing of tantalum powder around tantalum wire, thus forming a tantalum anode element; (b) sintering the pressed powder at high temperatures (1400-2000° C.) to form a sponge-like structure; (c) anodizing the sintered tantalum-coated tantalum conductor of step (b), thereby yielding Ta2O5 on the surface of particles and the anode (dielectric formation); (d) forming a MnO2 conductive layer (manganzing) on the formed Ta2O5; (e) dipping the element into a graphite dispersion and completing the cathode formation process by dipping into a silver dispersion
This is a rather cumbersome process that frequently yields a non-homogeneous, tantalum and niobium layer. Furthermore, the thickness of a metal particles layer formed by mechanical pressure is generally limited to over 800 microns. A thinner layer is not producible using conventional mechanical pressing procedures.
In order to improve this process and makes it more economical and technologically rational, the present invention is directed to providing a stable suspension for performing an electrophoretic step intended for replacing the above mentioned mechanical pressing step (a). The major advantage of applying an electrophoretic deposition step in this particular case lies in the possibility of controlling the homogeneity and the thickness of the metal layer formed in step (a).
EPD allows obtaining a thickness of deposit layers in the range from 10 microns to few millimeters. In contradiction, the conventional mechanical EPD allows obtaining a thickness of deposit layers in the range from 10 microns to few millimeters. In contradiction, the conventional mechanical pressing procedure cannot provide a metal particles layer of less than about 800 microns. Consequently, the EPD procedure according to the present invention reduces the capacitor overall size and the effective equivalent serial resistance (ESR).
Electrophoretic deposition processes are well known and are described, inter alia, in the U.S. Pat. Nos. 5,919,347, 6,059,949 and 6,127,283.
However, of the known prior art neither teaches how to obtain homogenously dispersed porous metal electrode applicable in a capacitor, wherein the porosity degree is in the range of 30 to 50% and the metal is capable of forming a stable, uniform, oxide layer having a dielectric constant greater than 25 (k≧25), preferably tantalum or niobium layer, with required thickness of about 20-500 micron, nor they teach how to obtain the required stable suspension for carrying out the electrophoretic process.
U.S. Pat. No. 4,067,735 discloses a method of making bulk porous anodes for electrolytic capacitors, which comprises preparing a suspension of tantalum or niobium powder and a binding material, and subjecting the suspension to an electric field established by a voltage applied to electrodes, whereby the metal powder and the binder deposit forming bulk porous bodies, which are then sintered at temperatures from 1600 to 2000° C., to produce bulk porous anodes. This process, however, does not permit to control the porosity of the final anodes nor to impart to them desired shapes without high temperature treatments, combined with shape controlling means. The bulk structure, also, is not optimal, and it would be desirable to control the shape of the final electrode without treatments in addition to the electrophoretic deposition.
Thus, there is a need for, and it is an aim of the present invention to provide a stable metal suspension, wherein said metal is capable of forming a stable oxide layer having a dielectric constant greater than 25 (k≧25), preferably selected from the group consisting of tantalum and niobium, for producing uniform deposition of metal particles in said metal layer, having a controllable range of layer thickness and porosity. It is a further aim of the present invention to provide such a selected from the group consisting of tantalum and niobium, comprising the step of electrophoretic uniform deposition of metal particles onto a substrate comprising said metal conductor.
It is a further object of present invention to provide a stable suspension for electrophoretic homogeneous deposition of said metal, preferably tantalum or niobium particles. A major aspect of the present invention is the use of such stable suspension in the electrophoretic deposition process for obtaining homogeneously dispersed tantalum or niobium particles in a substantially uniform thickness layer. This layer may have a wide range of thickness, from a few microns to millimeters, and may be obtained in several minutes.
It is an additional object of present invention to provide a metal, preferably tantalum or niobium anode for use in capacitors, wherein such an electrode comprises uniformly dispersed tantalum or niobium particles within said tantalum or niobium layer thereof.